Strong magnetoelectric coupling in Tb– Fe/ Pb „ Zr
0.52Ti
0.48… O
3thin-film heterostructure prepared by low energy cluster beam deposition
Shifeng Zhao, Yujie Wu, Jian-guo Wan,a兲XinWei Dong, Jun-ming Liu, and Guanghou Wang
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China,
and International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China 共Received 30 November 2007; accepted 11 December 2007; published online 10 January 2008兲 The magnetoelectric Tb– Fe/Pb共Zr0.52Ti0.48兲O3 thin-film heterostructure was prepared by low energy cluster beam deposition. The microstructures, ferroelectric property, leakage current, and magnetization, as well as magnetoelectric effect were investigated for the heterostructure. It is shown that the thin-film heterostructure displays the well-defined microstructure with clear interface. The heterostructure not only exhibits good ferromagnetic and ferroelectric properties, but also possesses strong magnetoelectric effect. The present work provides an ideal avenue to prepare magnetoelectric composite films and facilitates their applications on the microelectromechanical system devices. ©2008 American Institute of Physics.关DOI:10.1063/1.2831695兴
In recent years, magnetoelectric films have drawn a con- tinually increasing interest due to their potential applications in the microelectromechanical system共MEMS兲devices.1By growing composite films combined with piezoelectric and magnetostrictive materials, the strong magnetoelectric cou- pling effect could be achieved due to product property.
So far, much work has been done to prepare the composite films by combining perovskite ferroelectric oxides 关e.g., Pb共Zr0.52Ti0.48兲O3共PZT兲, BaTiO3兴with ferromagnetic oxides 共e.g., CoFe2O4, La0.67Sr0.33MnO3兲.2–5 Due to low magneto- striction of the ferromagnetic oxides, the reported magneto- electric effects in these all-oxide composite films are gener- ally not strong.
It is well known that R 共rare earth兲-Fe iron alloy pos- sesses giant magnetostriction, being an order of magnitude greater than the ferromagnetic oxides.6The previous investi- gations have shown that the magnetoelectric effect in the bulk laminate consisted ofR-Fe alloy and ferroelectric oxide is much larger than that of the all-oxide laminates.7–9There- fore, for the laminated composite thin film 共i.e., thin-film heterostructure兲, it could be expected that magnetoelectric effect would be enhanced significantly ifR-Fe alloy is used in the magnetostriction layer. However, by conventional film preparation means, since the phase-formation temperature of R-Fe alloy is very high 共the substrate is generally heated above 500 ° C兲, it is unavoidable to bring about serious oxy- gen diffusion from PZT oxide toR-Fe alloy. As a result, both magnetostriction inR-Fe alloy and piezoelectricity in PZT are seriously suppressed. Moreover, the serious oxygen dif- fusion would also generate a new interface layer, which fur- ther significantly decreases the magnetoelectric coupling efficiency.8Due to these factors, so far, few investigations on relative work have been carried out.
Recently, we have developed an effective preparation method, namely, low energy cluster beam deposition 共LECBD兲, to prepare the nanostructured Tb–Fe film,10which makes it possible to prepare the well-defined microstructured
thin-film heterostructure consisted ofR-Fe alloy and ferro- electric oxide. We have demonstrated that such nanostruc- tured Tb–Fe film possesses higher magnetostriction than the common Tb–Fe films prepared by other methods.10 More importantly, during LECBD process, the phase formation of Tb–Fe nanoclusters共or nanoparticles兲is achieved in the con- densation chamber with high temperature, while the deposi- tion of Tb–Fe nanocluster beam onto the substrate is achieved in another high vacuum chamber with low energy and low temperature 共e.g., room temperature兲. Both pro- cesses are completely independent of each other. Therefore, even if the substrate is ferroelectric oxide, the degree of the interfacial reaction or diffusion between Tb–Fe alloy and ferroelectric oxide would be greatly suppressed. In this letter, we report the preparation of Tb–Fe/PZT bilayer thin-film heterostructure by LECBD process. The well-defined micro- structured with clear interface is achieved in the heterostruc- ture, and strong magnetoelectric effect is observed.
A 100 nm thick PZT film deposited on the Pt/Ti/SiO2/Si wafer was used as the substrate in this work.
The details on the preparation of PZT thin film could be found elsewhere.11 The substrate was blocked by a mask with the open holes of 0.2 mm in diameter. A dc-magnetron- sputtering-gas-aggregation cluster source was used to pro- duce the cluster beam and a 5 cm diameter TbFe2alloy plate was used as the sputtering target. Both argon and helium gases were used as the condensed inert carrier gases for the nanoclusters. After passing through the skimmer, a high- orient Tb–Fe nanocluster beam forms. The Tb–Fe nanoclus- ter beam finally deposits onto the surface of the PZT film through the open holes of the mask. During LECBD process, the background pressure of the system was 4⫻10−5 Pa. The final thickness of the Tb–Fe layer was⬃300 nm. The details on LECBD process could be found elsewhere.10After depo- sition, not taking off the mask, a Pt electrode layer was de- posited on the Tb–Fe dots via pulse laser deposition. The structure of the thin-film heterostructure is sketched in inset 共a兲of Fig.1.
Figure 1 shows the surface scanning electron micros- copy共SEM兲image of the Tb–Fe layer in the heterostructure.
a兲Author to whom correspondence should be addressed. Electronic mail:
APPLIED PHYSICS LETTERS92, 012920
共
2008兲
0003-6951/2008/92共1兲/012920/3/$23.00 92, 012920-1 © 2008 American Institute of Physics Downloaded 15 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
It is seen that the Tb–Fe layer is compactly assembled by the regular spherical nanoparticles with average diameter of
⬃30 nm, which are distributed uniformly and adjacent with each other. Inset共b兲of Fig.1shows the cross-sectional SEM image of the thin-film heterostructure. One observes that the interface between Tb–Fe and PZT layers is clear and no tran- sition layer is observed. In addition, the x-ray diffraction result 共not shown here兲 also proved that there existed no additional phase peaks except PZT and Tb–Fe phase peaks.
The vertical-transport resistivity measurement for the heterostructure yields a resistivity of ⬃2.1⫻1010⍀cm at zero bias, indicating that the heterostructure is a very good dielectric insulator. Figure2共a兲presents the polarization ver- sus electric field共P-E兲 hysteresis loops for the heterostruc- ture measured by a RT66 ferroelectric testing unit with the
applied electric voltage of 5 – 15 V. The well-defined ferro- electric loops are observed. Under the applied electric volt- age of 15 V, the saturation polarization and remanent polarization are Ps= 51C/cm2 and Pr= 27C/cm2, re- spectively, both of which only have a very slight decrease compared with the pure PZT film 共Ps= 58C/cm2 and Pr= 34C/cm2兲. Such slight decrease in ferroelectric prop- erties of the heterostructure should be attributed to the in- crease of oxygen vacancy concentration in PZT layer, which brings about difficulty for the mobility of domain walls in a certain degree and further leads to the decrease in polarization.12
In order to further understand this, we performed the leakage current density measurement at room temperature, as shown in inset of Fig.2共a兲. It is clearly shown that the leak- age current density in the heterostructure is quite low, e.g., only being⬃1.5⫻10−4A/cm2 even under the higher elec- tric field of 30 MV/m. In spite of this, we found that the leakage current density in the heterostructure was still higher than that of the pure PZT film, which indicates the increase of free carrier density in PZT layer of the heterostructure.13It was reported that, under high electric field, the leakage cur- rent in the ferroelectric film was closely relative to the oxy- gen vacancies, which could be well explained by a conduc- tion mechanism based on Schottky emission model.14 Accordingly, we infer that there should appear induced oxy- gen vacancies in PZT layer near the interface during prepa- ration of the heterostructure, which obviously origins from the slight oxygen diffusion from PZT layer to Tb–Fe layer. In fact, though the LECBD process is under the low energy, the Tb–Fe nanoclusters still possess certain energy 共in the range of 0.01– 0.1 eV/atom兲. Due to the very strong oxidation activity for the rare-earth Tb–Fe alloy, some Tb–Fe nanoclusters with higher energy may have a chance to take oxygen from PZT layer when they land onto the PZT sub- strate, consequently, giving rise to the introduction of oxygen vacancies.
Figure 2共b兲 presents the field dependent magnetization 共M-H兲 curves for the heterostructure measured by a super- conducting quantum interference device. The heterostructure exhibits the well-defined magnetic hysteresis loops. One ob- serves that both in-plane and out-of-plane coercive fields are the same as onlyHc⬃60 Oe, much lower than that of the bulk Tb–Fe alloy, while the in-plane and out-of-plane satu- ration magnetizations are ⬃38 and ⬃47 emu/cm3, respec- tively. According to our previous work,10 the Tb–Fe nano- structured film prepared by LECBD process exhibits high magnetostriction under the low magnetic field, e.g., typically being⬃300⫻10−6under dc magnetic biasHbias= 3.5 kOe.
Since magnetoelectric effect in a two-phase composite mainly origins from the interfacial stress transfer between the magnetostrictive and the ferroelectric phases, such low coer- civity and high magnetostriction are significantly beneficial to the magnetoelectric coupling.11,15
We subsequently measured the magnetoelectric effect for the heterostructure. Both Hbias and small ac magnetic field 共10 Oe兲were applied along the film plane. The induced volt- age increment 兩⌬VME兩 was recorded by a lock-in amplifier 共SRS Inc., SR830兲. Figure 3 plots the Hbias dependence of 兩⌬VME兩 at a given ac magnetic field frequency f= 1.0 kHz.
One observes that the film exhibits strong magnetoelectric coupling. With increasingHbias, the兩⌬VME兩value rapidly in- creases, reaching the maximum value of 14V at Hbias
FIG. 1. 共Color online兲The surface SEM image of the Tb–Fe layer in the thin-film heterostructure. Inset共a兲is a sketch of the heterostructure and inset 共b兲is the fractured cross-sectional SEM image of the heterostructure.
FIG. 2. 共Color online兲 共a兲Polarization vs electric field hysteresis 共P-E兲 loops for the thin-film heterostructure. Inset is the variation of leakage cur- rent density with the applied electric field.共b兲The field dependent magne- tization共M-H兲curves for the thin-film heterostructure.
012920-2 Zhaoet al. Appl. Phys. Lett.92, 012920共2008兲
Downloaded 15 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
= 5.5 kOe, and then drops slowly. The calculated maximum increment of the magnetoelectric voltage coefficient is as high as ⬃140 mV/cm Oe, larger than that of the reported all-oxide ferroelectric-ferromagnetic composite film.2–4Inset of Fig.3further plots theHbiasdependence of piezomagnetic coefficientq共=␦/␦Hbias兲for the pure Tb–Fe nanostructured film prepared by LECBD process. Compared with these two curves, we find that both兩⌬VME兩 in heterostructure andq in Tb–Fe film have the similar change trend with Hbias. This indicates that the magnetoelectric coupling in the hetero- structure should be dominated by the magnetic-mechanical- electric transform through the stress-mediated transfer, which is well in agreement with the model proposed by Srinivasan et al.16In addition, we believe that the magnetoelectric effect in the heterostructure could be further enhanced by control- ling the oxygen diffusion in the interface, which could be achieved if the kinetic energy of the nanocluster beam is modulated appropriately. The relative investigations are now underway.
In summary, the Tb–Fe/PZT thin-film heterostructure was prepared by LECBD process. The heterostructure shows a well-defined microstructure with clear interface. The slight
oxygen diffusion between PZT and Tb–Fe layers was testi- fied by the leakage current density measurement. In spite of this, the heterostructure exhibits good ferroelectric and ferro- magnetic properties, and the strong magnetoelectric effect was observed in the heterostructure. The present work opens an ideal avenue to prepare the magnetoelectric composite films, which facilitates their applications on MEMS devices.
This work was financially supported by the National Natural Science Foundation of China共Grant Nos. 10774070, 90406024, and 90606002兲, the Provincial Nature Science Foundation of Jiangsu in China共BK2006123兲, the National Key Projects for Basic Research of China共2002CB613303 and 2006CB921802兲, and the Program for New Century Ex- cellent Talents in University of China共NCET-07-0422兲.
1N. A. Spaldin and M. Fiebig, Science 309, 391共2005兲.
2Y. G. Ma, W. N. Cheng, M. Ning, and C. K. Ong, Appl. Phys. Lett. 90, 152911共2007兲.
3J. P. Zhou, H. C. He, Z. Shi, and C. W. Nan, Appl. Phys. Lett. 88, 013111 共2006兲.
4H. Zheng, J. Wang, L. Mohaddes-Ardabili, M. Wuttig, L. Salamanca-Riba, D. G. Schlom, and R. Ramesh, Appl. Phys. Lett. 85, 2035共2004兲.
5W. Eerenstein, M. Wiora, J. L. Prieto, J. F. Scott, and N. D. Mathur, Nat.
Mater. 6, 348共2007兲.
6R. C. O’handley,Modern Magnetic Materials: Principles and Applica- tions共Wiley, New York, 2000兲.
7J. Ryu, A. V. Carazo, K. Uchino, and H. E. Kim, Jpn. J. Appl. Phys., Part 1 40, 4948共2001兲.
8C. W. Nan, G. Liu, and Y. H. Lin, Appl. Phys. Lett. 83, 4366共2003兲.
9G. Srinivasan, A. A. Bush, K. E. Kamentsev, V. F. Meshcheryakov, and Y.
K. Fetisov, Appl. Phys. A: Mater. Sci. Process. 78, 721共2004兲.
10S. F. Zhao, F. Bi, J. G. Wan, M. Han, F. Q. Song, J. M. Liu, and G. H.
Wang, Nanotechnology 18, 265705共2007兲.
11J. G. Wan, X. W. Wang, Y. J. Wu, M. Zeng, Y. Wang, H. Jiang, W. Q.
Zhou, G. H. Wang, and J. M. Liu, Appl. Phys. Lett. 86, 122501共2005兲.
12J. F. Scott and M. Dawber, Appl. Phys. Lett. 76, 3801共2000兲.
13C. Wang, M. Takahashi, H. Fujino, X. Zhao, E. Kume, T. Horiuchi, and S.
Sakai, J. Appl. Phys. 99, 054104共2006兲.
14K. Numata, Thin Solid Films 515, 2635共2006兲.
15T. Wu, M. A. Zurbuchen, S. Saha, R.-V. Wang, S. K. Streiffer, and J. F.
Mitchell, Phys. Rev. B 73, 134416共2006兲.
16G. Srinivasan, E. T. Rasmussen, J. Gallegos, R. Srinivasan, Yu. I. Bokhan, and V. M. Laletin, Phys. Rev. B 64, 214408共2001兲.
FIG. 3.共Color online兲TheHbiasdependence of the induced magnetoelectric voltage increment兩⌬VME兩at a given dc magnetic frequencyf= 1.0 kHz for the thin-film heterostructure. Inset is theHbiasdependence of piezomagnetic coefficientqfor the pure Tb–Fe nanostructured film prepared by LECBD process.
012920-3 Zhaoet al. Appl. Phys. Lett.92, 012920共2008兲
Downloaded 15 Mar 2010 to 219.219.118.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
